With certain types of internal combustion engines, high temperature operation, along with agitation of the gasoline in the fuel tank can cause high pressure buildups in the tank. The high pressure buildup can result in an overly rich gas-air mixture being supplied to the carburetor which in turn can produce stalling of the engine. This applies in particular to engines having a vacuum suction carburetion system, where the fuel is drawn directly from the tank to the carburetor by vacuum created by air flow through the carburetor throat. When this type of engine is operated at elevated temperatures, above 85.degree. F., for extended periods and when there is momentary agitation or sloshing of fuel in the tank, high pressure buildups can occur in the tank which must be rapidly vented. The pressure buildup can be aggravated in situations where the engine is enclosed by a hood, or in engines in which the exhaust system is located adjacent the fuel tank.
The high pressure buildup in the fuel tank, if not properly vented, can adversely effect engine performance. To vent the excessive pressure, gas caps have been provided in the past with vent passages or holes. The most common type of venting mechanism is a domed cap in which a dome is secured to the upper end of the cap to provide an upper chamber between the cap and the dome and a single central outlet is provided in the dome. In this type of construction, foam plastic has been located in the chamber to aid in separating liquid fuel from the vapor. However, under extreme pressure buildup conditions, liquid fuel is not adequately separated from the vapor so that liquid fuel may be discharged upwardly from the cap in the form of a liquid stream.